Insulin's complex interaction with specific binding sites in the plasma membrane of the adipose cell is accompanied by a rapid, concentration-dependent stimulation of glucose transport into, and glucose metabolism by, this cell. The mechanism of these actions of insulin are under investigation through 1) the development of a mathematical model of 125I-insulin binding, and 2) studies of the relationship between insulin binding and glucose transport activity under perturbed experimental conditions. To date, a one binding site model of insulin's interaction with the adipose cell has been rejected although some of the kinetics of insulin association can be successfully explained by the hormone degradation portion of this model. While a two binding site model can explain the kinetics of both association and dissociation at a given insulin concentration, the parameters of each site or the proportion of sites with constant parameters must vary with insulin concentration. A variety of experimental conditions have been found which appear to differentially affect insulin binding and insulin's ability to stimulate glucose transport, thereby providing several potential means of identifying that specific aspect of insulin-receptor interaction leading to enhanced glucose transport.